The power electronics technology which controls the main circuit current by using a power semiconductor element is applied in broad fields such as major appliances, a railroad and an electric vehicle.
In late years, as the power semiconductor element, the element to which more efficient materials such as SiC (Silicon Carbide) or GaN (Gallium Nitride) are applied attracts attention in addition to the IGBT (Insulation Gate type Bipolar Transistor) and the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) which are used conventionally.
Regarding these power semiconductor elements, principally, a gate electrode and an emitter electrode are formed on the upper surface thereof, and a collector electrode is formed on the lower surface thereof.
As the configuration of the power semiconductor device in which the power semiconductor element is included, generally, the configuration in which the power semiconductor element is joined directly to a lead frame (electrode) using a solder is known.
However, in a power semiconductor device of such a configuration, since a coefficient of linear expansion of the lead frame (electrode) and a coefficient of linear expansion of the power semiconductor element are greatly different, large heat stress is applied repeatedly to the solder joining part by a power cycle at the time of making the power semiconductor device drive, finally, a solder crack occurs and a problem of joining failure arises.
Recently, in order to cope with such a problem to meet the requirement of the securing of reliability for the power cycle at high temperature, a pressed-contact type semiconductor device has attracted attention, which realizes an electrical contact by performing a direct surface contact without the solder between the lead frame (electrode) and each one of the gate electrode and the emitter electrode, which are formed on the upper surface of the power semiconductor element, and the collector electrode, which is formed on the lower surface of the power semiconductor element, and always applying pressure.
When this pressed-contact type power semiconductor device is used, the heat stress at the time of the power cycle occurs on a connection boundary surface between the electrode of the power semiconductor element and the lead frame (electrode). However, since the connection part is not a metallic fixed connection such as the solder joining but a contact connection, the connection boundary surface thereby slides, the heat stress is eased and it is possible to prevent the connection part from breaking down, and then it becomes possible to realize a connection with high reliability.
However, in the case of such a pressed-contact type power semiconductor device, it is difficult to realize a surface contact between the electrode of the semiconductor element and the lead frame (electrode) with uniform pressure, due to a processing dimensional variation, a curve, and an undulation of various components such as a lead frame (electrode), a heat radiation plate, and the like which constitute the device. As a result, there is a problem that the connection resistance value is not stabilized, and the characteristic of the device becomes unstable.
FIG. 9 is a schematic sectional view showing the problem of such a pressed-contact type power semiconductor device.
FIG. 9 shows, for instance, a case in which a gap K occurs between an upper electrode 105 formed on the upper surface of the power semiconductor element 101 and a lead frame (electrode) 104, and a gap L occurs between a lower electrode 106 formed on the lower surface of the power semiconductor element 101 and a lead frame (electrode) 102 because the thickness of the lead frame (electrode) 102 varies on the electrode plane of the power semiconductor element 101, that is, G>H, and also the thickness of the lead frame (electrode) 104 varies on the electrode plane of the power semiconductor element 101, that is, J>I. As a result, even if the applied pressure A is always applied perpendicularly in the thickness direction of the power semiconductor element 101, the uniform surface contact between the lower electrode 106 of the power semiconductor element 101 and the lead frame (electrode) 102 and between the upper electrode 105 of the power semiconductor element 101 and the lead frame 104 is not realized, and it becomes difficult to obtain a stable connection resistance value.
In the case where the power semiconductor device is constructed by using the components with a general tolerance, there is an example in which gaps K and L of 10 μm or more occur.
A way of coping with such a case by making the processing finished dimensions of the various constitutional components strictly equal could be considered. However, this leads to an increase in the cost of the components, and it is actually difficult to cope with the case by such a way.
For such a problem, as a method for absorbing the processing dimensional variation of various constitutional components, a method of allowing a soft metal sheet made of Ag or the like to be intercalated, as a variation correction plate, between the electrode of the power semiconductor element and the lead frame (electrode) is proposed (see, for example, Japanese Patent Application Laid-open No. H08-88240).
FIG. 10 shows a sectional structure of the joining part of the power semiconductor device in which the soft metal sheet is intercalated between the electrode of the power semiconductor element and the lead frame (electrode).
The lower electrode 106 of the power semiconductor element 101 such that at least the upper electrode 105 is formed on the upper surface thereof and the lower electrode 106 is formed on the lower surface thereof is, with the applied pressure A being always applied, electrically connected with the lead frame (electrode) 102 via the soft metal sheet 103, which is disposed between the lower electrode 106 and the lead frame (electrode) 102 facing the lower electrode 106.
Incidentally, in the case of the configuration shown in FIG. 10, the upper electrode 105 which is formed on the upper surface of the power semiconductor element 101 and the lead frame (electrode) 104 are in direct contact with each other, however, the contact configuration, in which the soft metal sheet 103 is intercalated therebetween, can be utilized.
The soft metal sheet 103 shown in FIG. 10 plays the role in correcting the gap which occurs between the lower electrode 106 of the power semiconductor element 101 and the lead frame (electrode) 102 due to the variation of the components, because the soft metal sheet 103 is deformed in the thickness direction when the pressure is applied to it.